The present invention relates to a combination instrument for a motor vehicle, in particular for a utility vehicle or a bus, having an LC display.
The design of combination instruments is known through their varied use in motor vehicles. They usually contain a plurality of display devices accommodated on a carrier plate in a single housing and inform the driver about a variety of operating states of the motor vehicle. The carrier plate is frequently embodied as an electrical printed circuit board. In addition to analog display devices, combination instruments increasingly also contain an LC display, i.e. a liquid crystal display. In a utility vehicle or in a bus the combination instrument which is usually of flat design is installed in dashboard at an inclined angle of typically 20xc2x0 to 50xc2x0 with respect to the horizontal, which has the advantage of permitting arrangements of instruments and electronic displays, thus also an LC display, over a large area, and thus in a particularly easily viewed way.
In contrast to the customary installation conditions of a combination instrument in a car when the combination instrument is usually installed in a virtually vertical position, the protection against glare, for example in the form of a hood which screens the combination instrument, is generally not provided in a utility vehicle or in a bus, with the result that ambient light can be incident directly onto the front of the combination instrument in a utility vehicle or in a bus. However, when that is the case, intensive light such as sunlight can adversely affect the legibility of displays on the LC display to a considerable extent. The adverse effect on the legibility of displays on the LC display results from the fact that unimpeded incidence of bright interfering light onto the LC display inevitably reduces the contrast of the representations of information on the LC display in relation to the ambient brightness.
The object of the present invention is then to disclose a combination instrument for a motor vehicle, in particular for a utility vehicle or a bus, having an LC display in which, despite the usual installation conditions for a combination instrument in a utility vehicle or in a bus and the associated interfering light influences, the legibility of information represented on the LC display is improved by increasing the contrast.
The object is achieved by means of the features of the present invention. The solution is featured by a light-guiding plate provided in the combination instrument,
into which ambient light is injected from inside or from outside the motor vehicle,
in which the injected ambient light is guided by means of total reflection at the side of the LC display facing away from the viewer, and
which has, at the side of the LC display facing away from the viewer, a coating or a structure for extracting the ambient light out of the light-guiding plate there and for injecting the ambient light into the LC display.
Further solution features are
that the LC display is of transmissive design,
that the light-guiding plate is composed of plastic,
that the coating of the light-guiding plate is white and highly reflective,
that analog display devices with scales are also provided in the combination instrument, and that the light-guiding plate has a structure or coating which is suitable for the selective extraction of light both in the region of the scales for their divisions and/or division indications, and in the region of the display face of the LC display,
that a photosensor is provided which, without being directly influenced by the ambient light, simply senses the intensity of the light present in the light-guiding plate, that, furthermore, LEDs are provided which inject their light into the light-guiding plate and that the intensity of the light emitted by the LEDs is controlled as a function of the light sensed by the photosensor,
that the LEDs used in conjunction with the photosensor emit white light for a transition from daylight operation to night-time operation which is as neutrally colored as possible,
that the light-guiding plate extends out of the housing of the combination instrument up to the windshield of the motor vehicle, as a result of which light which is incident into the motor vehicle through the windshield can be injected into the light-guiding plate with minimum possible obstruction,
that the light-guiding plate which extends out of the housing of the combination instrument is embedded in the dashboard of the motor vehicle, and the dashboard has, in the region in front of the windshield, an opening for the injection of the ambient light into the light-guiding plate,
that the light-guiding plate is provided in the region in front of the windshield with suitable structures or a coating which reduces the refractive index, said structures or coating promoting the injection of the ambient light into the light-guiding plate.
The increase in the contrast according to the invention has the advantage of obviating the need for additional active electrical light sources. A combination instrument which is equipped with the abovementioned features
a) does not have a higher electrical power demand, b) does not require a larger number of electrical components, c) does not produce any waste heat which is unavoidable with electrical light sources, and d) can be implemented in a cost-effective way because the light-guiding plate can be fabricated in virtually any desired shape as an injection-molded component. The suggested approach to a solution is based on the fact that the incidence of light which is per se an interference factor is used for backlighting the LC display. By virtue of the fact that the instant light beams are collected, guided behind the LC display and caused to emerge there in a concentrated fashion, it is possible to increase the contrast of the representations on the LC display at just the point when an increase in the contrast is most urgently required, specifically when there is intensive ambient light, for example during the day when bright sunlight is incident.